This invention relates to a constant-voltage circuit and more particularly, to a constant-voltage circuit suitable for a battery checking device.
In battery-powered electronic devices, such as desk-top calculators and electronic time keeping devices, it is required to incorporate a battery checking device for displaying that the battery life is coming to an end. FIG. 1 shows a prior art battery checker circuit with CMOS transistors.
With the prior art circuit, since the gm's of a p-channel transistor T.sub.P1 having a threshold voltage V.sub.T and an n-channel transistor T.sub.N1 with the gate supplied with a sampling pulse .phi. of a fixed frequency are large, the potential V.sub.1 at the junction between the resistors R.sub.1 and R.sub.2 connected with the gate of a p-channel transistor T.sub.P2 is approximately (V.sub.DD -V.sub.T).times.R.sub.2 /R.sub.1 +R.sub.2. As the battery voltage drops, the gate-to-source voltage of the transistor T.sub.P2 is reduced and gm of the transistor T.sub.P2 decreases. The drop rate of the potential V.sub.2 at the junction of the transistor T.sub.P2 and a variable resistor R.sub.3 connected to the input of an inverter I is greater than that of V.sub.1 due to the amplifying effect of the transistor T.sub.P2, and also is greater than the drop rate of the threshold voltage V.sub.th of the inverter I which is approximately half the supply voltage. The circuit is so designed that the potential V.sub.2 is higher than the threshold voltage V.sub.th of the inverter and hence the output of the inverter I is at a low level (V.sub.SS) when the battery voltage is at a full level with the result that an indicator L connected between the output of the inverter I and the terminal V.sub.SS is turned off. Since the drop rate of V.sub.2 is greater than that of the threshold voltage V.sub.th of inverter I while the battery voltage is dropping, V.sub.2 becomes lower than V.sub.th when the supply voltage drops below a predetermined level. Consequently, the output level of the inverter I becomes high (V.sub.DD) and the indicator L is caused to light.
In the aforementioned circuit the threshold voltages of the inverter I and the transistor T.sub.P2 vary depending on the manufacturing process, so that the variable resistor R.sub.3 is indispensible to the adjustment of V.sub.2. Further, in this circuit, both V.sub.1 and V.sub.2 increase with the rise in temperature, so that the indicator L does not often light even if the battery voltage drops below the predetermined level.
It is desired that the battery checker circuit be not affected by variation in the threshold voltage of the MOS transistors or by temperature change. For checking the end of battery life, it is advisable to compare a voltage varying at a fixed rate as the battery voltage drops with a reference voltage which may vary at the same rate with the battery voltage. This reference voltage is required hardly to change with temperature.